Various methods are known for the use of plasma melt burners for plasma melt cutting. In some of these procedures the plasma melt cutting burner operates under normal atmospheric conditions ("dry plasma"), or the cutting process takes place under water ("water plasma"). When plasma melt cutting burners are employed under normal atmospheric conditions, the arc between the workpiece and the cathode is restricted through a nozzle to achieve the energy density required for cutting. Plasma melt cutting burners of this type employ a carrier gas for the production of the plasma jet, such as an argonhydrogen mixture, nitrogen, oxygen, or air. High quality cutting can be achieved through this method, with reasonably good utilization of the amount of energy used. However, such burners produce a high noise level, emit a dazzling light, toxic gases, dust as well as vaporized metals.
When the cutting procedure takes place under water the foregoing, hygienically harmful conditions can be avoided. Although water has a detrimental effect on the quality of the cutting process, it absorbs a part of the harmful materials, and reduces the noise level and the intensity of the emitted light.
The principle of water injection was developed for improving the quality of the cut. In these cases water is injected in the nozzle orifice of a plasma melt cutting burner and an additional water curtain is used for reducing harmful materials and light effect.
The injected water evaporates, partially dissociates under the effect of the energy of the plasma jet, and also protectively surrounds the same. During this procedure a rotation of the plasma jet takes place. As a result, the two cut edges which are obtained of dissimlar quality; one cut edge is of good quality, whereas the other is of poor quality. In the case of cutting of shaped parts one must assure that the qualitatively better cut edge is in the formed workpiece.
It is not always possible to have the water provide sufficient protection of the cutting station and the plasam jet. This becomes especially significant when the distance between the plasma burner and the workpiece cannot be maintained constant.
In a different kind of burner a ring-shaped nozzle surrounds the plasma melt cutting burner. The plasma jet here encloses a cylindrical or conical bell shaped water formation. A flow of gas is created within this bell shaped water formation. This impacts conically onto the cutting site and an increased inner pressure is formed within the bell shaped water formation. In this case an amount of gas in the range of from about 0.057 to about 0.566 m.sup.3 /min is required for maintaining sufficient protection of the plasma arc with increased inner pressure within the bell shaped water formation. This kind of operation requires a costly, complex burner, complicated associated apparatus, and a high degree of servicing.
A substantial drawback of plasma melt cutting burners operating with water is the reduction of the cutting velocity in comparison to operating a dry plasma under otherwise identical circumstances. A plasmatron is described in German Federal Republic published patent application No. 3,514,851. This nozzle is formed so that it enables the simultaneous introduction of inert, oxygen-containing and plasma-forming gas as well as water, and independently from each other. This should reduce the required amount of inert gas. The water which exits the slit-like opening with a twist forms a protective conical water cover of variable shape in front of the nozzle. The exiting water is used mainly for cooling of the heated part of the nozzle. The gases are introduced into the nozzle as a vortex, therefore the plasma jet will rotate.
The conical water umbrella formed by the nozzle enables the reduction of the noise and blinding effect of the light, but provides insufficient protection of the plasma jet from the water in the case of cutting a material under water. A further drawback is the extremely complicated and expensive structure of the nozzle. Furthermore, two different cutting edges are formed due to the rotation of the plasma jet. Also the materials dissolved in the cooling water tend to settle on the surfaces of the parts to be cooled, and this reduces the useful life of these parts.